Investments (Schedule of Unrealized Loss on Investments) (Detail) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	$ 5.2	$ 70.5
12 months or less, Fair Value	471.1	2,221.5
Greater than 12 months, Gross Unrealized Losses	32.0	24.2
Greater than 12 months, Fair Value	870.2	227.3
Total, Gross Unrealized Losses	37.2	94.7
Total, Fair Value	1,341.3	2,448.8
Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	4.4	64.7
12 months or less, Fair Value	440.5	2,102.4
Greater than 12 months, Gross Unrealized Losses	29.4	21.9
Greater than 12 months, Fair Value	841.8	203.7
Total, Gross Unrealized Losses	33.8	86.6
Total, Fair Value	1,282.3	2,306.1
Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.8	3.0
12 months or less, Fair Value	20.6	73.9
Greater than 12 months, Gross Unrealized Losses	1.8	1.9
Greater than 12 months, Fair Value	25.0	22.9
Total, Gross Unrealized Losses	2.6	4.9
Total, Fair Value	45.6	96.8
Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	5.2	67.7
12 months or less, Fair Value	461.1	2,176.3
Greater than 12 months, Gross Unrealized Losses	31.2	23.8
Greater than 12 months, Fair Value	866.8	226.6
Total, Gross Unrealized Losses	36.4	91.5
Total, Fair Value	1,327.9	2,402.9
US Treasury and Government [Member] | Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		12.3
12 months or less, Fair Value	6.4	247.9
Greater than 12 months, Gross Unrealized Losses	6.3	1.9
Greater than 12 months, Fair Value	152.0	18.8
Total, Gross Unrealized Losses	6.3	14.2
Total, Fair Value	158.4	266.7
Foreign Government Debt Securities [Member] | Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.8	1.5
12 months or less, Fair Value	100.7	129.0
Greater than 12 months, Gross Unrealized Losses	0.5	0.1
Greater than 12 months, Fair Value	56.1	17.3
Total, Gross Unrealized Losses	1.3	1.6
Total, Fair Value	156.8	146.3
Municipal [Member] | Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.4	14.8
12 months or less, Fair Value	53.3	345.3
Greater than 12 months, Gross Unrealized Losses	5.7	4.3
Greater than 12 months, Fair Value	158.9	39.9
Total, Gross Unrealized Losses	6.1	19.1
Total, Fair Value	212.2	385.2
Municipal [Member] | Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.3
12 months or less, Fair Value	4.4
Total, Gross Unrealized Losses	0.3
Total, Fair Value	4.4
Corporate Debt Securities [Member] | Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	2.6	21.4
12 months or less, Fair Value	198.4	872.7
Greater than 12 months, Gross Unrealized Losses	10.1	11.6
Greater than 12 months, Fair Value	259.7	87.7
Total, Gross Unrealized Losses	12.7	33.0
Total, Fair Value	458.1	960.4
Corporate Debt Securities [Member] | Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.5	2.9
12 months or less, Fair Value	16.2	71.9
Greater than 12 months, Gross Unrealized Losses	1.8	1.6
Greater than 12 months, Fair Value	25.0	21.4
Total, Gross Unrealized Losses	2.3	4.5
Total, Fair Value	41.2	93.3
Residential Mortgage Backed Securities [Member] | Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.5	10.3
12 months or less, Fair Value	39.7	321.1
Greater than 12 months, Gross Unrealized Losses	5.3	3.4
Greater than 12 months, Fair Value	135.5	29.5
Total, Gross Unrealized Losses	5.8	13.7
Total, Fair Value	175.2	350.6
Residential Mortgage Backed Securities [Member] | Below Investment Grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		0.1
12 months or less, Fair Value		2.0
Greater than 12 months, Gross Unrealized Losses		0.3
Greater than 12 months, Fair Value		1.5
Total, Gross Unrealized Losses		0.4
Total, Fair Value		3.5
Commercial Mortgage Backed Securities [Member] | Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses	0.1	4.2
12 months or less, Fair Value	31.3	155.4
Greater than 12 months, Gross Unrealized Losses	1.4	0.6
Greater than 12 months, Fair Value	65.4	10.2
Total, Gross Unrealized Losses	1.5	4.8
Total, Fair Value	96.7	165.6
Asset-backed Securities, Securitized Loans and Receivables [Member] | Investment grade
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		0.2
12 months or less, Fair Value	10.7	31.0
Greater than 12 months, Gross Unrealized Losses	0.1
Greater than 12 months, Fair Value	14.2	0.3
Total, Gross Unrealized Losses	0.1	0.2
Total, Fair Value	24.9	31.3
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
12 months or less, Gross Unrealized Losses		2.8
12 months or less, Fair Value	10.0	45.2
Greater than 12 months, Gross Unrealized Losses	0.8	0.4
Greater than 12 months, Fair Value	3.4	0.7
Total, Gross Unrealized Losses	0.8	3.2
Total, Fair Value	$ 13.4	$ 45.9